Coanda Gas Burner Apparatus and Methods

ABSTRACT

A gas burner apparatus for discharging a mixture of fuel gas, air and flue gas into a furnace space of a furnace wherein the mixture is burned and flue gas having a low content of nitrous oxides and carbon monoxide is formed is provided. The burner tile includes at least one gas circulation port extending though the wall of the tile. The interior surface of the wall of the tile includes a Coanda surface. Fuel gas and/or flue gas conducted through the gas circulation port follows the path of the Coanda surface which allows more flue gas to be introduced into the stream. The exterior surface of the wall of the tile also includes a Coanda surface for facilitating the creation of a staged combustion zone. Also provided are improved burner tiles, improved gas tips and methods of burning a mixture of air, fuel gas and flue gas in a furnace space.

This Application is a Division of application Ser. No. 11/454,071, filedJun. 14, 2006

BACKGROUND OF THE INVENTION

The present invention relates to gas burner apparatus and methods ofburning fuel gas in the presence of air and furnace flue gas to create astable flame while suppressing the formation of nitrous oxides(“NO_(x)”) and carbon monoxide (“CO”).

Various types of gas burners have been developed and successfullyutilized with a combination of both diffusion and pre-mix capabilities.A pre-mix approach mixes both the air and fuel gas to a homogenousmixture prior to combustion within the confines of the furnace. Adiffusion approach injects the fuel gas into an air stream whereinmixing takes place without a venturi. The flame is stabilized close tothe point of exit, creating both thermal and prompt nitrous oxides. Bothapproaches are routinely utilized to ignite and combust a given fuel gasto generate heat within a process burner.

In both pre-mix and diffusion-type burners, an optimum approach can bedefined to reduce both thermal and prompt nitrous oxide formation. Theemission of nitrous oxide and carbon monoxide gases by process burnersas well as other combustion equipment is closely regulated by thegovernment. The government is constantly pushing for better methodologyto further reduce emissions from current combustion equipment.

In order to lower the production of nitrous oxides and other potentiallypolluting gases, various improved gas burner apparatus are beingdeveloped. In one approach, all of the air together with primary fuel isburned in a first zone and the remaining fuel is burned in a secondzone. In this staged fuel gas approach, the staged fuel becomes dilutewith furnace flue gas diluting a substantial portion of the gas streamduring combustion thereby lowering the combustion temperature of thegases. The nitrogen in the air and flue gas functions as a heat sink inthat it absorbs heat from the flame. The flue gas can come from thefurnace stack (external flue gas) or from the furnace itself (internalflue gas). Lowering the combustion temperature of the gases lowers theformation of nitrous oxides in the produced flue gases. Examples of lowNO_(X) burners and associated methods are shown by U.S. Pat. No.5,275,552 (issued to John Zink Company on Jan. 4, 1994) and U.S. Pat.No. 6,729,874 B2 (issued to John Zink Company on May 4, 2004), which areincorporated by reference herein.

Staged combustion and dilution of the fuel gas create additionalconcerns that need to be addressed, including non-combustibility andflame instability. An appreciable amount of air or flue gas is needed todilute the flame enough to achieve a sufficient reduction in nitrousoxide formation. However, if the fuel gas is overly diluted, it may bedifficult to ignite or the ignited flame may become unstable. Flameinstabilities can create further instabilities capable of destabilizingthe entire furnace.

Coanda surfaces have been utilized in flares wherein significant flowrates at elevated pressures are a reality. A Coanda surface is merely acurved surface designed for the adherence of a fluid. Fluid streamsinjected on or adjacent to a Coanda surface tend to adhere to and followthe path of the surface. The negative pressure and viscous forces pullthe fluid against the surface. The fluid stream is spread into arelatively thin film or sheet, which allows proximate fluids to be mixedin with the fluid stream in a very efficient manner. The additionalsurface area imparted to the gas significantly enhances mixing. In aflare, for example, which may emit tens of thousands of pounds of wastegas per hour, fast mixing is desirable. As a result, Coanda surfaces andthe Coanda effect are commonly used in flare apparatus as it eliminatesthe need for steam, blowers and related equipment.

However, Coanda surfaces have not been incorporated into low NO_(X)process burner apparatus. Burner components are smaller and entail muchlower gas flows than flare components. As a result, Coanda technologyhas not been actively applied to process burners. Also, many refineryoperators have not changed the refinery furnaces due to the expenseinvolved therewith. As a result, replacement burner assemblies oftenhave to fit into existing furnace boxes which defines the performancecriteria the burner must meet (for example, the length and diameter ofthe flame).

By the present invention, various ways have been discovered to utilizeCoanda surfaces in low NO_(X) staged fuel gas burners to greatly improvethe efficiency of the burners while avoiding problems such asnon-combustibility and flame instability.

SUMMARY OF THE INVENTION

In accordance with the present invention, gas burner apparatus andmethods are provided which meet the needs described above and overcomethe deficiencies of the prior art. It has been discovered that a Coandasurface can be coupled with a free fluid stream to mix fuel gas with airand a diluent (furnace flue gas in this case) while maintaining extendedturndown capabilities and enhanced stability. The Coanda surface greatlyenhances mixing of the flue gas with the other fluids in the stream.Further, by the use of various Coanda surfaces, the amount of flue gasthat can be incorporated into a mixing zone and flame can be greatlyincreased. Thus, the ability to reduce nitrous oxide and carbon monoxideemissions from the burner can be greatly increased while improving flamequality and heat flux distribution in the furnace. The Coanda surfacesand the way the surfaces are positioned on the inside and outside of theburner tile allow the flue gas to be imparted to various mixing andcombustion zones associated with the burner without diluting the fuelgas on the inner boundary layer to a point that it becomesnon-combustible or results in an instable flame. The Coanda surfacesalso allow the shape of the flame to be accurately controlled withoutthe need for other structures such as flame-holders, cones, wings,impingement plates and so forth. These and other advantages of theinvention are described in detail below.

In accordance with one aspect of the invention, a gas burner apparatusis provided for discharging a mixture of fuel gas and air into a furnacewherein the mixture is burned in the presence of flue gas whileproducing a low content of nitrous oxides and carbon monoxide. The gasburner apparatus comprises a plenum, a burner tile, primary fuel gasinjection means, and secondary fuel gas injection means. A pre-mixprimary means of injection can also be included in the apparatus.

The plenum includes a housing for attachment to the furnace. The housingincludes an upper end attached to the furnace, the upper end having anair outlet disposed therein, a lower end opposing the upper end, and asidewall connecting the upper end and the lower end together. At leastone of the sidewall and the lower end has an air inlet disposed therein.

The burner tile has a central opening therein for receiving air from theair outlet of the housing. The burner tile includes a bottom endattached to the upper end of the housing over the air outlet, a top endopposing the bottom end, the top end including a discharge outlet, and awall connecting the bottom end to the top end and surrounding thecentral opening. The wall extends into the furnace and has an interiorsurface, an exterior surface and at least one gas circulation portextending through the wall, the interior surface of the wall includingan internal Coanda surface which bulges into the central opening. Theinternal Coanda surface is positioned on the interior surface of thewall adjacent to (preferably over) the gas circulation port.

The primary fuel gas injection means is connected to a source of fuelgas and operably associated with the burner apparatus for injectingprimary fuel gas into the central opening of the burner tile. Theprimary fuel gas injection means includes an outer gas riser connectedto the source of fuel gas, the outer gas riser having an outer primaryfuel gas discharge nozzle connected thereto and positioned outside ofthe wall of the burner tile to inject primary fuel gas through the gascirculation port into the central opening of the tile. The primary fuelgas injection means can also include various other components.

In one embodiment, the primary fuel gas injection means includes apre-mix unit. The pre-mix unit combines a pre-mix membrane and a venturimixer. The pre-mix membrane extends around the interior surface of thewall of the burner tile below the gas circulation port therein and has aplurality of pre-mix gas discharge orifices (“ports”) in the topthereof. The venturi mixer includes an inner gas riser connected to thesource of fuel gas and having an inner primary fuel gas discharge nozzleconnected thereto, and a venturi housing operably associated with theinner gas riser and primary fuel gas discharge nozzle. The venturihousing is connected to the pre-mix membrane for feeding a mixture ofprimary fuel gas and air into the pre-mix membrane. The pre-mix unit iscapable of delivering a range of lean mixtures of primary fuel gas andair into the central opening of the burner tile.

The secondary fuel gas injection means is connected to a source of fuelgas and operably associated with the burner apparatus for injectingsecondary stage fuel gas from outside the burner tile to a pointadjacent to the discharge outlet of the burner tile (preferably on oradjacent to the exterior surface of the burner tile). The secondary fuelgas injection means includes an outer gas riser connected to the sourceof fuel gas and having a secondary fuel gas discharge nozzle connectedthereto for injecting secondary fuel gas on or adjacent to the exteriorsurface of the wall of the burner tile. In one configuration, theprimary fuel gas injection means and secondary fuel gas injection meansutilize the same outer gas riser and fuel gas discharge nozzle. The fuelgas discharge nozzle serves as both the primary fuel gas dischargenozzle and the secondary fuel gas discharge nozzle. The nozzle includesone or more ports for injecting fuel gas through the gas circulationport extending through the wall of the burner tile and one or more portsfor injecting fuel gas on or adjacent to the exterior surface of thewall of the burner tile.

The exterior surface of the wall of the burner tile preferably alsoincludes an external Coanda surface which bulges outwardly from theexterior surface. The outer gas riser and secondary fuel gas dischargenozzle injects secondary stage fuel gas on or adjacent to the externalCoanda surface. The external Coanda surface preferably extendscompletely around the exterior surface of the wall of the burner tile;however, it can also intermittently extend around the exterior surfaceof the wall of the burner tile. The intermittent external Coandasurfaces are preferably spaced by external planar surfaces which can bevertical or inclined inwardly toward the central opening of the tile.

In another embodiment, the gas burner includes a plenum, a burner tile,primary fuel gas injection means and secondary fuel gas injection means.The plenum includes a housing for attachment to the furnace. The housingincludes an upper end attached to the furnace, the upper end having anair outlet disposed therein, a lower end opposing the upper end, and asidewall connecting the upper end and the lower end together. At leastone of the sidewall and the lower end has an air inlet disposed therein.

The burner tile has a central opening therein for receiving air from theair outlet of the housing. The burner tile includes a bottom attached tothe upper end of the housing over the air outlet, a top end opposing thebottom end, the top end including a discharge outlet, and a wallconnecting the bottom end to the top end and surrounding the centralopening. The wall extends into the furnace space and has an interiorsurface and an exterior surface, the exterior surface of the wallincluding an external Coanda surface which bulges outwardly from theexterior surface.

The primary fuel gas injection means is connected to a source of fuelgas and operably associated with the burner apparatus for injectingprimary fuel gas into the central opening of the burner tile. Thesecondary fuel gas injection means is also connected to a source of fuelgas and operably associated with the burner apparatus for injectingsecondary stage fuel gas from outside of the burner tile to a pointadjacent to the discharge outlet of the burner tile. The secondary fuelgas injection means includes an outer gas riser connected to the sourceof fuel gas and having a secondary fuel gas discharge nozzle connectedthereto for injecting secondary stage fuel gas on or adjacent to theexternal Coanda surface.

In another aspect, the present invention includes burner tiles for usein association with a burner plenum to form a gas burner apparatus fordischarging a mixture of fuel gas and air into a furnace wherein themixture is burned in the presence of flue gas while producing a lowcontent of nitrous oxides and carbon monoxide. The inventive burnertiles are the burner tiles described above in association with theinventive gas burner apparatus. The inventive burner tiles can be usedin retrofit applications.

In another aspect, the invention includes a gas tip for use inassociation with a gas burner apparatus. The gas tip comprises a gasbarrel for connection to a source of fuel gas, a gas deflector attachedto the gas barrel, and a fuel gas outlet disposed between the gas barreland the gas deflector. The gas deflector has an exterior surface thatincludes a Coanda surface positioned with respect to the fuel gas outletsuch that fuel gas discharged from the fuel gas outlet follows the pathof the Coanda surface. The gas deflector preferably has a tulip shape.The inventive gas tip can be used, for example, as the secondary stagefuel gas discharge nozzle of the inventive gas burner apparatus, as thetip of a pilot for the inventive gas burner apparatus or as a primaryinner fuel gas discharge nozzle attached to a central inner gas riser(for example, a central gas gun). The inventive gas tip can also be usedin connection with a series of gas nozzles serving as primary gas tipsaround the inner perimeter of the tile.

In another aspect, the invention provides a method of burning a mixtureof air and fuel gas in the presence of flue gas in a furnace to generateheat in the furnace wherein a gas burner apparatus having a mixing zonefor mixing the air, fuel gas and flue gas prior to combustion thereof isutilized. The method comprises the following steps:

-   -   (a) providing a Coanda surface in the mixing zone;    -   (b) injecting fuel gas on or adjacent to the Coanda surface in a        manner that entrains flue gas from outside the mixing zone into        the mixing zone and causes the flue gas to mix with the air and        fuel gas in the mixing zone;    -   (c) discharging the mixture of combustion air, fuel gas and flue        gas from the mixing zone into the furnace; and    -   (d) burning the mixture of combustion air, fuel gas and flue gas        discharged from said mixing zone in the furnace.

In one embodiment, the mixing zone is surrounded by a wall and themixture of air, fuel gas and flue gas is discharged from the mixing zoneinto a primary reaction zone in the furnace. In this embodiment, themethod further comprises the steps of:

-   -   (e) providing an external Coanda surface on the exterior surface        of the wall; and    -   (f) injecting a stream of secondary stage fuel gas on or        adjacent to the external Coanda surface in a manner that        entrains flue gas into the stream to create a secondary fuel        gas/flue gas mixture and causes the secondary fuel gas/flue gas        mixture to burn in a secondary reaction zone in the furnace.

In another embodiment, the inventive method comprises the steps of

-   -   (a) providing a Coanda surface on the exterior surface of the        wall of the burner apparatus;    -   (b) injecting primary fuel gas into the mixing zone in a manner        that causes the fuel gas to mix with air in the mixing zone;    -   (c) discharging the mixture of air and fuel gas from the mixing        zone; and    -   (d) burning the mixture of air and fuel gas discharged from the        mixing zone in a primary reaction zone in the furnace;    -   (e) injecting a stream of secondary stage fuel gas on or        adjacent to the external Coanda surface in a manner that        entrains flue gas into the stream to create a secondary fuel        gas/flue gas mixture and causes the secondary fuel gas/flue gas        mixture to burn in a secondary reaction zone in the furnace.

The interior surface of the wall of the burner apparatus preferably alsoincludes an internal Coanda surface. The fuel gas injected into themixing zone is injected on or adjacent to the internal Coanda surface ina manner that entrains flue gas from outside the mixing zone into themixing zone and causes the flue gas to mix with the air and fuel gas inthe mixing zone.

The objects, features and advantages of the present invention will bereadily apparent to those skilled in the art upon a reading of thedescription of preferred embodiments which follows when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a section view of the gas burner apparatus of the presentinvention attached to a furnace floor.

FIG. 2 is a perspective view of the burner tile of the gas burnerapparatus of the present invention.

FIG. 3 is a section view of the burner tile of the gas burner apparatusof the present invention.

FIG. 3A is a section view similar to FIG. 3 and further illustrating agas circulation choke that can be incorporated into the inventive burnertile.

FIG. 4 is an enlarged detail view of a portion of the burner tileillustrated by FIG. 3 illustrating the flow of gas in association withthe burner tile.

FIG. 4A is an enlarged detail view of a portion of the burner tile ofFIG. 3A illustrating the flow of gas in association with the burnertile.

FIG. 4B is an enlarged detail view of another portion of the burner tileshown by FIG. 4.

FIG. 5 is a sectional view taken along the line 5-5 of FIG. 2.

FIG. 6 is a sectional view taken along the line 6-6 of FIG. 2.

FIG. 7 is another detail view of a portion of the burner tile shown byFIG. 3 illustrating a portion of the pre-mix unit.

FIG. 8 is a section view similar to FIG. 1 but illustrating the use of acentral venturi mixer in lieu of the gas gun shown by FIG. 1.

FIG. 9 is a section view similar to FIGS. 1 and 8 but illustrating theuse of a plurality of internal gas risers in lieu of the pre-mix unit.FIG. 9 also illustrates the use of a conventional pilot in associationwith the inventive gas burner apparatus.

FIG. 10 is a section view of the burner tile illustrated by FIG. 3 butillustrating a different outer gas riser configuration.

FIG. 11 is a section view illustrating an alternative embodiment of theinventive burner tile.

FIG. 11A is a section view taken along the line 11A-11A of FIG. 12 andillustrating one variation of the planar wall sections (inclined) of theburner tile of FIG. 11.

FIG. 11B is a section view taken along the line 11B-11B of FIG. 12 andillustrating another variation of the planar wall sections(straight/vertical) of the burner tile of FIG. 11.

FIG. 12 is a section view taken along the line 12-12 of FIG. 11.

FIG. 13 is a section view illustrating yet another embodiment of theinventive burner tile.

FIG. 14 is an enlarged detail view of a portion of the burner tile shownby FIG. 13.

FIG. 15 a section view taken along the line 15-15 of FIG. 13.

FIG. 16 is a section view illustrating yet another embodiment of theinventive burner tile.

FIG. 17 is an enlarged detail view of a portion of the burner tile ofFIG. 16.

FIG. 18 is a section view taken along the line 18-18 of FIG. 16,

FIG. 19 is a section view taken along the line 19-19 of FIG. 16.

FIG. 20 is a section view illustrating yet another embodiment of theinventive burner tile.

FIG. 21 is a section view taken along the line 21-21 of FIG. 20.

FIG. 22 is a partial section view illustrating the inventive gas tip asconfigured for use as a pilot.

FIG. 23 is an enlarged detail view of a portion of the gas tipillustrated by FIG. 22.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, and particularly to FIG. 1, the gasburner apparatus of the present invention is illustrated and generallydesignated by the numeral 10. As shown by FIG. 1, the burner apparatus10 is sealingly attached to a furnace wall 12 (preferably the bottomwall or floor) of a furnace space 14 of a furnace 16 (the overallfurnace is not shown) over an opening 18 in the wall. Although gasburner apparatus are commonly mounted vertically and fired upwardly asshown in FIG. 1, it is to be understood that the gas burner apparatus 10can also be mounted in other ways. For example, the gas burner apparatus10 can be mounted horizontally and fired horizontally or vertically, orcan be mounted vertically and fired downwardly (down-fired). Preferably,the gas burner apparatus 10 is vertically mounted to the floor of thefurnace space 14 and up-fired as shown in the drawings.

The gas burner apparatus 10 discharges a mixture of fuel gas and airinto the furnace space 14 of the furnace 16 wherein the mixture isburned in the presence of flue gas while producing a low content ofnitrous oxides and carbon monoxide. The gas burner apparatus 10comprises a plenum 20 including a housing 22 for attachment to thefurnace. The housing includes an upper end 24, a lower end 26 opposingthe upper end and a sidewall 28 connecting the upper end and lower endtogether. The upper end 24 of the housing 22 has an air outlet 30disposed therein. As shown by FIG. 1, the upper end 24 of the housing 22is attached to the furnace wall 12 such that the air outlet 30 ispositioned underneath the opening 18 in the furnace wall. At least oneof the sidewall 28 and the lower end 26 of the housing 22 has an airinlet 32 disposed therein. Preferably, and as shown by FIG. 1, the airinlet 32 is disposed in the sidewall 28 of the housing 22.

As illustrated by FIG. 1, the housing 22 is attached to the bottom wallor floor 12 of the furnace 16 by means of a flange 34 and a plurality ofbolts 36 which extend through complementary openings 38 in the flangeand bottom wall of the furnace. The furnace wall 12 includes an internallayer of insulating material 40 attached thereto. An air flow registeror damper 42 for regulating the rate of flow of air through the airinlet 32 is attached to the air inlet. The damper 42 includes aplurality of adjustable fins 44 which can be rotated from vertical tohorizontal to open and close the damper. A muffler 46 for reducing bothjet and combustion noise is also attached to the air inlet 32. Asunderstood by those skilled in the art, the gas burner apparatus 10 canbe a natural draft burner (i.e., the air required for combustion isnaturally drafted into the housing 22), a forced draft burner (forexample, a blower is used to blow the combustion air into the housing),a balanced draft burner (for example, blowers are used to both blow airin and blow air out of the burner to achieve an appropriate balance ofcombustion air) or variations thereof. A variety of different types offuel gas can be burned by the burner apparatus 10, including naturalgas, hydrogen, propane, ethane or other typical refinery-type fuels.

The gas burner apparatus 10 further comprises a burner tile 50 having acentral opening 52 therein for receiving air from the air outlet 30 ofthe housing 22. The burner tile 50 includes a bottom end 54, a top end56 opposing the bottom end and a wall 58 connecting the bottom end tothe top end and surrounding the central opening 52. The bottom end 54 ofthe burner tile 50 is attached to the upper end 24 of the housing 22over the air outlet 30 of the housing. The top end 56 of the burner tile50 includes a discharge outlet 60 therein.

Referring now to FIGS. 1-6, the wall 58 of the burner tile 50 extendsinto the furnace space 14 and has an upper portion 62, a lower portion64, an interior surface 66 and an exterior surface 68. The wall 58further includes a plurality of gas circulation ports 70 extendingthrough the wall. The interior surface 66 of the wall 58 includes aplurality of internal Coanda surfaces 80 positioned adjacent to or over(over as shown) the gas circulation ports 70, each internal Coandasurface bulging into the central opening 52 of the burner tile 50. Eachinternal Coanda surface 80 and gas circulation port 70 are positioned ina recessed section 82 in the interior surface 66 of the wall 58. Eachrecessed section 82 includes opposing sidewalls 84 and 86 that extendfrom the interior surface 80 of the wall 58 into the central opening 52.As best shown by FIG. 4B, the sidewalls 84 and 86 extend further intothe central opening 52 than the internal Coanda surface 80 that ispositioned in the corresponding recessed section 82 extends into thecentral opening. Put another way, the internal Coanda surfaces 80 areinset into the interior surface 66 of the wall 58. The internal Coandasurfaces 80 are preferably inset in the interior surface 66 of the wall58 by a distance in the range of from about 0.25 inches to about 0.75inches. As described further below, the space between the internalCoanda surfaces 80 and the interior surface 66 of the remaining portionof the wall 58 prevents fuel gas and/or flue gas from being swept off ofthe internal Coanda surfaces by the flow of fuel gas and/or air throughthe central opening 52 of the burner tile 50.

In order to achieve a significant Coanda effect, the surfaces of theinternal Coanda surfaces 80 should be substantially smooth and have asubstantially true radius or uniform arc. Also, it is important for eachinternal Coanda surface to have enough curvature to sufficiently attractthe gas stream at issue. If the Coanda surface does not have enoughcurvature or surface area, the surface may not have a sufficient area toinitiate the Coanda effect due to the momentum of the gas (i.e., the gasstream may not be drawn to the surface). In order to assure a sufficientCoanda effect, the ratio of the diameter of the fuel discharge port thatinjects fuel gas into and through the gas circulation port 70 on oradjacent to the subject internal Coanda surface 80 (or average portdiameter if multiple fuel discharge ports are used (the “primary portdiameter”) to the radius of the internal Coanda surface (the “internalCoanda radius”) needs to be at least 7:1. For example, the diameter ofthe port (or average diameter if multiple ports are involved) of theprimary fuel gas discharge nozzle 166 to the internal Coanda radiusneeds to be at least 7:1. Preferably the primary port diameter tointernal Coanda radius ratio is at least 10:1, most preferably at least12:1. So, for example, with a primary port diameter of 0.0625 inches anda 0.75 inch internal Coanda radius, the primary port diameter tointernal Coanda ratio is 12:1.

Assuming that the Coanda surface has enough curvature or surface area,the gas stream or jet is aligned to be tangent with the curvature of theCoanda surface to initiate a proper Coanda effect, even when dealingwith small gas ports. This can vary significantly with large Coandasurfaces used in flares, for example, where higher mass flows inconjunction with a slotted injection scenario are utilized.

Apart from the above parameters, the particular size and shape of theinternal Coanda surfaces 80 can vary depending on the size and shape ofthe gas circulation ports, the size and shape of the burner tile andother factors relating to the particular application. The orientation ofthe internal Coanda surfaces 80 (e.g., vertical, horizontal, etc.) onthe interior surface 66 can also vary depending on the above factors.

The internal Coanda surfaces 80 are a very important component of theinventive gas burner 10. They allow a great deal of flue gas to beentrained without overly diluting the fuel gas and preventing combustionor causing flame instability. This is at least partly due to the innerboundary layer remaining fuel rich. The stream of primary fuel gas andair injected through the gas circulation ports 70 is pulled andmaintained against the Coanda surfaces 80. The fuel gas stream is brokenapart and expanded into a film containing a much broader surface area.The center of the core of gas is exposed. As a result, the distance andtime needed to mix the flue gas with the fuel gas (and any other fluidsinvolved in the particular application, for example air and/or steam) issubstantially lessened. Significantly more flue gas and air (and otherfluids if desired) can be mixed with the fuel gas jet. As a result, amore stable flame is created, the content of nitrous oxides in the fluegas generated by the burner is reduced and the flame can be more easilyshaped.

As shown by FIGS. 3A and 4A, in one configuration the burner tile 50further includes circulation choke means 87 positioned in the gascirculation ports 70 for inhibiting the flow of air from within thecentral opening 52 of the burner tile 50 through the gas circulationports to outside of the tile. The circulation choke means 87 includes ashield 88 for each gas circulation port 70. The shields 88 are attachedto the wall 58 of the burner tile 50 and extend upwardly into thecorresponding gas circulation port 70. As shown, the shields 88 can bean integral part of the refractory burner tile. The circulation chokemeans 87 is used in applications in which it is necessary to abate theflow of fluids from inside the tile through the gas circulation ports 70to outside the tile. Outbound fluid flow can occur, for example, when adiffusion jet stream is not injected through the circulation gas ports70. Elimination of the outflow of air through the ports 70 aids inreducing emissions and adds flame shaping capabilities to the design(when diffusion jets are not injected through the ports 70 to maintain afluid seal between the two fluid flow regimens). The circulation chokemeans 87 prevent the air from short circuiting the tile and therebyraising nitrous oxide emissions, and keep the flame off the exteriorsurface of the wall of the burner tile. The circulation choke means 87also stop any premature interaction between the pre-mixed gas and thediffusion gas in the central opening 52. In some cases, without theshield 88 in place, the momentum of the diffusion primary will pull thepre-mix flame into the circulation port 70 where it then carries airprematurely to the base of the diffusion jet.

The entire burner tile 50 including the shield 88 (when the shield isutilized) is made of a heat and flame resistant refractory material,that is, a material that has the ability to retain its physical shapeand chemical identity even when subject to high temperatures. Examplesof refractory materials that can be used include silicon carbide,alumina mixtures and ceramic fiber materials.

Referring now specifically to FIGS. 4, 4A and 4B, the gas circulationports 70 are illustrated in detail. Each gas circulation port 70includes a ledge 90, a top surface 92 (which is a portion of theinternal Coanda surface 80) and a pair of opposing sidewalls 94 and 96interconnecting the ledge and top surface together. When the burner tiledoes not include the circulation choke means 87, as shown by FIG. 4, theledges 90 are either flat, that is, substantially co-planar with the topsurfaces 92 of the ports 70, or inclined downwardly from the interiorsurface 66 toward the exterior surface 68 of the wall 58. Preferably,the ledges 90 are inclined downwardly from the interior surface 66toward the exterior surface 68 of the wall 58 at an angle in the rangeof from 15° to −60°. For example, when the outer gas risers (discussedbelow) do not substantially extend through the wall 12 of the furnace,the ledges 90 are inclined downwardly at a greater angle. Inconfigurations in which the outer gas risers substantially extend abovethe bottom wall 12 of the furnace, the ledges 90 are inclined downwardlyat an angle, for example, in the range of about 10° to about 60°.Preferably, the ledges 90 are inclined downwardly from the interiorsurface 66 toward the exterior surface 68 of the wall 58 at an angle inthe range of from 15° to 25°. When the burner tile 50 includes thecirculation choke means 87, as shown by FIG. 4A, the ledges 90 inclinedownwardly from the interior surface 66 toward the exterior surface 68of the wall 58 at a fairly severe angle due to the presence of theshield 88 in the gas circulation port 70. The downward incline of theledges functions to prevent air inside the central opening 52 fromradially exiting the central opening 52 through the ports 70. Whether ornot the circulation choke means 87 is used and the angle at which theledge 90 inclines will depend on the particular application.

The interior surface 66 of the upper portion 62 of the wall 58 furtherincludes a primary bluff body 100 which has a flat surface 102 facingupwardly, that is facing the discharge outlet 60 of the burner tile. Theprimary bluff body 100 extends completely around the interior surface 66of the wall 58. Each of the internal Coanda surfaces 80 includes a lowerend 104, an upper end 106 and a bulge portion 108 connecting the lowerend and upper end together. The lower ends 104 of the internal Coandasurfaces 80 extend over the top of the gas circulation ports 70. Theupper ends 106 of the internal Coanda surfaces 80 terminate at the flatsurface 102 of the primary bluff body 100. The top end 56 of the burnertile 50 includes a secondary bluff body 110 which has a flat surface 112facing upwardly, that is facing the furnace space 14. The secondarybluff body 110 extends completely around the interior surface 66 of thewall 58. The primary bluff body 100 creates a low pressure zone andprovides a mixing zone in the upper portion of the central opening 52.The secondary bluff body 110 functions to stabilize the gas at thedischarge outlet 60 of the tile 50. Staged fuel has the ability toenrich the stabilized fuel on the top end 56 of the tile 50 in the eventit becomes too lean or diffuse,

The exterior surface 68 of the wall 58 of the burner tile 50 includes aplurality of port sections 116 (which include a gas circulation port 70)and a plurality of non-port sections 118 (which do not include a gascirculation port 70). The upper portion 62 of exterior surface 68 of thewall 58 of the burner tile 50 also includes an external Coanda surface130 which bulges outwardly from the exterior surface 68.

In one embodiment, as shown by FIGS. 1-10, the external Coanda surface130 extends completely around the exterior surface 68 of the wall 58.This scenario allows all of the staged fuel to be shaped by a Coandasurface.

In another embodiment, as shown by FIGS. 11-12, the upper portion 62 ofexterior surface 68 of the wall 58 of the burner tile 50 includes aplurality of external Coanda surfaces 130, each bulging outwardly fromthe exterior surface 68. In the embodiment shown by FIGS. 11-12, theexternal Coanda surfaces 130 are spaced apart by external planarsurfaces 132. As shown by FIGS. 11A and 11B, the external planarsurfaces 132 can be either inclined toward the central opening 52 of theburner tile (FIG. 11A) or straight or vertical (substantially parallelto the longitudinal axis of the burner tile) (1 IB). If inclined, theexternal planar surfaces 132 slope inwardly at an angle in the range offrom 5° to 25°. The use of alternating external Coanda surfaces andplanar (flat) surfaces (inclined or straight) provide for more controlwith respect to the shape of the flame. The staged fuel can be moreaggressively shaped to maintain a narrow flame. This is especiallyimportant where effects of the wall 58 need to be overcome. A portion ofthe fuel gas can be injected at aggressive angles to further enhanceflame shaping, or allow for a more aggressive biasing of the stagedfuel.

In yet another embodiment, as shown by FIGS. 13-15, the upper portion 62of exterior surface 68 of the wall 58 of the burner tile 50 includes anexternal planar surface 134 that extends completely around the exteriorsurface 68 of the wall 58. The external planar surface 134 slopesinwardly at an angle in the range of from 5° to 25°. It can also besubstantially straight or vertical (not inclined inwardly). Thisembodiment allows the staged drillings to be much more aggressive toallow significant capabilities to be realized within the shaping of theflame.

Thus, the various configurations of the upper portion 62 of the exteriorsurface 68 of the burner tile 50 allow the size and shape of the flameto be accurately controlled depending on the application. Additionaladvantages are achieved as well.

In order to achieve a significant Coanda effect, the surfaces of theexternal Coanda surfaces 130 should be substantially smooth and have asubstantially true radius or uniform arc. Also, it is important for eachexternal Coanda surface to have enough curvature to sufficiently attractthe gas stream at issue. If the Coanda surface does not have enoughcurvature or surface area, the surface may not have a sufficient area toinitiate the Coanda effect due to the momentum of the gas (i.e., the gasstream may not be drawn to the surface). In order to assure a sufficientCoanda effect, the ratio of the diameter of the fuel discharge port thatinjects fuel gas on or adjacent to the subject external Coanda surface130 (or average port diameter if multiple fuel discharge ports are used)(the “secondary port diameter”) to the radius of the external Coandasurface (the “external Coanda radius”) needs to be at least 7:1. Forexample, the diameter of the port (or average diameter if multiple portsare involved) of the secondary fuel gas discharge nozzle 166 to theexternal Coanda radius needs to be at least 7:1. Preferably thesecondary port diameter to external Coanda radius ratio is at least10:1, most preferably at least 12:1.

Apart from the above parameters, the particular size and shape of theexternal Coanda surfaces 130 can vary depending on the size and shape ofthe burner tile and other factors relating to the particularapplication. The orientation of the external Coanda surfaces 130 (e.g.,vertical, horizontal, etc.) on the exterior surface 68 can also varydepending on the above factors.

The external Coanda surface(s) 130 are also a very important componentof the inventive gas burner apparatus 10. The surface(s) 130 function toentrain more flue gas into the staged fuel gas stream and greatlyenhance the mixing process. When combined with the more conventionalexternal planar surfaces 132 or surface 134, the external Coandasurface(s) allow a great deal of precision and flexibility in achievingthe type and degree of staged combustion needed for the particularapplication. The external Coanda surface(s) 132 enhances the diluting ofthe fuel gas jet while maintaining a stable flame. If desired, theexternal Coanda surface(s) 132 can be used in connection with theinventive burner tile 50 when the tile does not have gas circulationports 70 therein.

In yet another embodiment, as shown by FIGS. 16-19, the burner tile 50further comprises a lip 140 transversely extending from the interiorsurface 66 of the wall 58 into the central opening 52 of the burnertile. The lip 140 is attached to the wall 58 adjacent to the top end 56of the burner tile 50 and extends around the interior surface 66 of thewall. The lip 140 includes a lower end 142, a top end 144 and a body 146connecting the lower end and top end together. The body 146 includes aplurality of protrusions 150 extending into said central opening 52 ofthe burner tile. The protrusions 150 include various cross-sectionalshapes (for example, elliptical, square, and triangular) and areseparated by grooves 152. As best shown by FIG. 19, the lower end 142 iscurved which facilitates the flow of fluids under the lip 140. Theoverall lip 140 functions to turn the fluid flow 90°. The fluid becomesvery dilute with air; the flame speed becomes low. The protrusions 150and grooves 152 cause the gas to stabilize and help maintain the flamein the event stabilization is needed due to over-dilution of fuel. Theradial projections serve as a bluff body to catch the lean mixture andstabilize it on the tip of the tile surface. This geometry can alsofunction with the central gas gun 170 or central venturi mixer 176 toprovide an enhanced quench mechanism to the flame.

Depending on the application, the gas burner apparatus 10 can includeboth the internal Coanda surfaces 80 and external Coanda surface(s) 130.Preferably, the gas burner apparatus 10 includes both the internalCoanda surfaces 80 and external Coanda surface(s) 130.

The gas burner apparatus 10 further comprises primary fuel gas injectionmeans 160 and secondary fuel gas injection means 162. The primary fuelgas injection means 160 are connected to a source of fuel gas (notshown) and operably associated with the burner apparatus 10 forinjecting primary fuel gas into the central opening 52 of the burnertile 50. The secondary fuel gas injection means 162 are connected to asource of fuel gas (not shown) and operably associated with the burnerapparatus 10 for injecting secondary stage fuel gas from outside of thecentral opening 52 and burner tile 50 to a point adjacent to thedischarge outlet 60 of the burner tile. As used herein and in theappended claims, primary fuel gas merely means fuel gas injected intothe central opening 52 of the burner tile (that is, any gas injectedinto the combustion zone formed by the confines of the burner tile 50).Secondary stage fuel gas merely means the fuel gas injected on theoutside or over the wall 58 of the burner tile 50.

The primary fuel gas injection means can include a variety of componentswhich can be used separately or together depending on the particularapplication.

As a first component, the primary fuel gas injection means 160 includesa plurality of outer gas risers 164 connected to a source of fuel gas.Each outer gas riser 164 has an outer primary (diffusion) fuel gasdischarge nozzle 166 (including one or more gas ports therein) connectedthereto which is positioned outside of said wall 58 of said burner tileto inject primary fuel gas through a gas circulation port 70 on oradjacent to the internal Coanda surfaces 80. The primary fuel gas ispreferably injected directly on to the internal Coanda surfaces 80. Asused herein and in the appended claims, a “nozzle,” for example a “fuelgas discharge nozzle,” is any kind of gas tip (typically connected to agas riser) that includes one or more gas discharge openings (forexample, ports or slots) therein for discharging or injecting a gasstream or jet from the nozzle. As used herein and in the appendedclaims, injection of a fluid (fuel gas in this case) “on or adjacent toa surface” means injection of the fluid directly on to the surface or inclose enough proximity to the surface for the surface to have an effect(for example, a Coanda effect) thereon. For example, it is sufficient ifthe fuel gas stream or jet is injected in close enough proximity to thecurvature of the Coanda surface for the Coanda effect to be initiated bythe pressure of the stream or jet in conjunction with the surface areaof the curved surface. In applications in which the temperatureassociated with the burner apparatus 10 is very high (for example, 2000°F. and above), the outer gas risers 164 do not substantially extendabove the wall 12 of the furnace in order to prevent damage thereto. Inother applications, both the risers 164 and nozzles 166 extend throughand above the wall 12.

As another component, the primary fuel gas injection means 160 can alsoinclude one or more inner gas risers 167, each inner gas riser beingconnected to a source of fuel gas and being positioned inside of theburner housing 22. Each inner gas riser has an inner primary fuel gasdischarge nozzle 168 (including one or more gas ports therein) connectedthereto for injecting primary stage fuel gas directly into the centralopening 52 of the burner tile. The use of a plurality of inner gasrisers 167 and inner primary fuel gas discharge nozzles 168 to injectfuel gas directly into the central opening 52 of the burner tile 50 isshown by FIG. 9. As shown, one or more risers 167 and correspondingnozzles 168 can be positioned at each gas circulation port 70 to injecta fraction of the primary fuel gas directly on or adjacent to aninternal Coanda surface 80, to help stabilize the flame.

As shown by FIGS. 1, 3 and 3A, an inner gas riser 167 and correspondinginner fuel gas discharge nozzle 168 can be used to form a central gasgun 170. An inner gas riser 167 is connected to a source of fuel gas andextends into the center of the central opening 52 of the burner tile 50.An inner fuel gas discharge nozzle 168 in the form of a bull nose tip(including a plurality of gas ports therein) is connected to the innergas riser 167. A gas dispersion cone 172 is attached to the centralriser and extends around the bull nose tip 168 for dispersing the gasdischarged by the tip. The central gas gun 170 can be used to inject afree jet of primary fuel gas directly into the burner tile 50. Themomentum of the free jet of primary fuel gas together with the momentumof the air pulls flue gas into the central opening 52 of the burner tile50 which helps reduce harmful emissions.

As shown by FIG. 8, an inner gas riser 167 and corresponding inner fuelgas discharge nozzle 168 can also be used to form a central venturimixer 176. An inner gas riser 167 is connected to a source of fuel gasand is positioned inside of the burner housing 22. An inner fuel gasdischarge nozzle 168 in the form of a gas spud (including one or moregas ports therein) is connected to the inner gas riser 167. A venturihousing 178 is operably associated with the riser 167 and nozzle 168.The venturi housing 178 is attached to the inner gas riser 167 andpositioned above the spud 168 for receiving the fuel gas discharged fromthe spud. The venturi housing 178 includes an inlet 180, an outlet 182and a venturi body 184 having a narrow portion 186 therein. The venturibody 184 creates a low pressure zone which entrains air into the housing178. A mixture of fuel gas and air is formed in the housing 178. Thecentral venturi mixer can be used to inject a pre-mix stream of primaryfuel gas and air directly into the burner tile 50. It creates a lean oreven ultra lean pre-mix zone to reduce flame length and further reducenitrous oxide emissions. Multiple venturi mixers 176 can be utilized ifdesired.

As shown by FIGS. 1, 3, 3A, 7 and 8, the primary fuel gas injectionmeans 160 can also include a pre-mix unit 190 which extends into thecentral opening 52 of the burner tile 50. As best shown by FIG. 7, thepre-mix unit 190 includes a pre-mix membrane 192 extending around andinset somewhat (for optimum stability) in the interior surface 66 of thewall 58 of the burner tile 50 below the gas circulation ports 70 in thewall. A plurality of pre-mix gas ports 194 is disposed in the top of themembrane 192. A pair of venturi mixers 196 feed pre-mix streams of fuelgas and air into the membrane 192. Each venturi mixer 196 includes aninner gas riser 198 connected to a source of fuel gas and having aninner primary fuel gas discharge nozzle 200 in the form of a gas spud(which includes one or more gas ports therein) connected thereto. Aventuri housing 202 is operably associated with the riser 198 and nozzle200. The venturi housing 202 is attached to the riser 198 and positionedto receive fuel gas discharged from the nozzle 200. The venturi housing202 includes an inlet 204, an outlet 206 and a venturi body 208,preferably having a narrow portion 210 therein. In some applications,the narrow portion 210 is not necessary. The venturi body 208 creates alow pressure zone which entrains air into the housing 202. A mixture offuel gas and air is formed in the housing 202 and conducted into thepre-mix membrane 192. The pre-mix unit 190 can be used to inject apre-mix stream of primary stage fuel gas and air around the perimeter ofthe interior surface 66 of the wall 58 of the burner tile 50.

The pre-mix unit 190 can serve as the total pre-mix primary or a partialpre-mix with the rest made up with diffusion primary fuel gas. Thepre-mix can be fixed heat release or modulated heat release like therest of the burner. The pre-mix unit 190 delivers the fuel symmetricallyaround the inside perimeter of the wall 58 of the tile 50 for enhancedturndown and stability. It also helps reduce nitrous oxide emissions dueto the homogenous delivery of air and fuel gas which reduces the basalcore temperature that would typically be observed with a diffusion typefree jet. When the pre-mix unit 190 is utilized in conjunction with adiffusion approach, the diffusion jets can be run much more dilute,and/or detached, as the diffusion flame will then be flame stabilized bythe pre-mix flame, which is lean. Since the diffusion jets are flamestabilized, the gas circulation ports 70 can be increased in flow areato a point in excess of six (6) times what would normally be achievablewithout negatively impacting flame stability (the flame is stabilized bythe pre-mix flame from the pre-mix unit). The pre-mix unit can be heldat a constant heat release. This allows this zone to be designed suchthat flashback is not a problem over the range of fuels. This allows notonly enhanced turndown due to flame stabilization, but also ensures thata lower primary is achieved while maintaining acceptable port sizing.This means a primary zone heat release can be achieved with as little asone percent (1%) of the total fuel in the primary zone. Due to thelarger gas circulation ports, carbon monoxide (CO) emissions can beminimized during cold startup scenarios. The appreciably larger gascirculation ports pull significant flue gases into the burner where theCO is re-burned to reduce the fractions of CO observed in the furnacebox.

The pre-mix unit 190 also supplies an ignition source for the remainingburner combustion zones. It can take many shapes and port quantities asrequired for the specific application. It can be adjusted by design togenerate a fuel gas-air mixture that is as lean as necessary to furtherreduce nitrous oxide emissions. The premix unit 190 serves as theminimum heat release for the burner such that a low heat releasedecoking cycle can be accomplished if necessary without affecting flamestability. The main gas delivery components can be turned off with theexception of the pre-mix unit. It then serves to deliver a very smallheat release while maintaining stability. When the main portion of theburner is relit, the pre-mix unit can then be brought back on-line atvery low pressures, much lower than would be typically possible.

The secondary fuel gas injection means 162 includes a plurality of outergas risers, each connected to a source of fuel gas and having asecondary fuel gas discharge nozzle (including one or more portstherein) connected thereto. The secondary fuel gas injection meansserves to inject secondary stage fuel gas on or adjacent to the exteriorsurface 68 (for example, the external Coanda surface(s) 130) of the wall58 of the burner tile 50. The secondary stage fuel gas is preferablyinjected directly on to the exterior surface 68 (for example, theexternal Coanda surface(s) 130). Various configurations of risers andnozzles can be utilized. For example, as shown by FIGS. 1, 4 and 4A, theouter gas risers and secondary fuel gas discharge nozzles of thesecondary fuel gas injection means are also the outer gas risers 164 andnozzles 166 of the primary fuel gas injection means. The nozzles 166include both primary ports that inject primary fuel gas into the gascirculation ports 70 and secondary ports that inject secondary stagefuel gas on or adjacent to the exterior surface 68 (for example, theexternal Coanda surface(s) 130) of the wall 58 of the burner tile 50. Inanother configuration, each outer gas riser 164 includes separateprimary fuel gas discharge nozzles and secondary fuel gas dischargenozzles. In yet another configuration, as shown by FIG. 10, the primaryfuel gas injection means and secondary fuel gas injection means utilizeseparate outer gas risers. A plurality of outer gas risers 164, eachconnected to a source of fuel gas and having an outer primary fuel gasdischarge nozzle 166 (including one or more gas ports therein) connectedthereto, are used to inject primary fuel gas through the gas circulationports 70 into the central opening 52 of the burner tile 50. Separateouter gas risers 214, each connected to a source of fuel gas and havinga secondary fuel gas discharge nozzle 216 (including one or more gasports therein) connected thereto, are used to inject secondary stagefuel gas on or adjacent to the exterior surface 68 (for example, theexternal Coanda surface(s) 130) of the wall 58 of the burner tile 50.The particular riser configuration utilized will depend on the amount ofgas staged, and the shape required of the flame.

The burner housing 22 and burner tile 50 preferably have circular orround cross-sectional shapes as shown in the drawings. However, thehousing 22 and burner tile 50 can have other shapes as well. Forexample, the housing 22 and burner tile 50 can have an elliptical,square or rectangular cross-sectional shape. The shape can besymmetrical or non-symmetrical as long as the Coanda surfaces areemployed correctly. The shape of the housing 22 does not need to be thesame as the shape of the burner tile 50. FIGS. 20 and 21 illustrate aburner tile 50 having a rectangular cross-sectional shape. Therectangular burner tile 50 can be used to generate a flat flame and isuseful in wall fired type applications, for example.

As shown by FIG. 1, except for the pre-mix unit 190, the variouscomponents of the primary fuel gas injection means 160 and secondaryfuel gas injection means 162 are connected to a burner gas header 217which is in turn connected to a source of fuel gas (for example, theoverall furnace header). The gas burner header 217 includes a headerinlet 218 and a plurality of header outlets 219 and associated headervalves 220. The pre-mix unit 190, specifically the inner gas risers 198thereof are preferably directly connected to a separate source of fuelgas (for example, from the overall furnace gas header). The risers 198are typically interconnected by a conduit 220 which is connected to theseparate source of fuel gas. The conduit 220 has a valve 222 disposedtherein for controlling the flow rate of fuel gas through the conduit.Connection of the pre-mix unit 190 to an independent source of fuelallows the pre-mix unit 190 to be operated at a fixed pressure whileserving as the burner primary. It also allows the flow rate of themixture of fuel gas and air from the pre-mix unit to be increased to apoint such that it is not necessary to inject primary fuel gas thoughthe gas circulation ports 70 if such a configuration is needed. Ifdesired, the pre-mix unit can also be connected to the burner gas header218 with merely a separate connector.

As shown by FIG. 9, the gas burner apparatus 10 can also compriseconventional pilot means 223 for igniting the primary fuel gas in theburner tile 50. The pilot means 223 includes an inner gas riser 226attached to a source of fuel gas, a venturi mixer 228 attached to theinner gas riser and a gas tip 230 (including one or more ports therein)attached to the venturi mixer. The gas tip 230 extends into the centralopening 52 of the burner tile. A shield 232 is positioned around the gastip to stabilize the pilot flame by ensuring the proper stoichiometry byadding additional air and protection to the flame. As shown by thearrows in FIG. 9, air is drawn in through the ports in the shield 232.The flame is discharged out of the top of the shield.

As stated above, the particular configuration of the gas burnerapparatus 10 including the configuration of the burner tile 50 and theset ups of the primary and secondary fuel gas injection means 160 and162 can vary depending on the application. In most instances, both theinternal Coanda surfaces 80 and external Coanda surfaces(s) 130 will beutilized. Regardless of the particular configuration utilized, theintent is to mix a great deal of flue gas with the fuel gas and airwithout negatively impacting the stability of the flame. The Coandasurfaces allow a new tool to be applied to flue gas entrainment andmixing, flame shaping and gas delivery. The enhanced mixing provided bythe Coanda surfaces results in improved heat flux, enhanced flamequality and enhanced heat delivery to the bottom of the furnace (flux).The staged fuel and secondary combustion zone serves to reduce emissionsof nitrous oxides and allows the flame to be shaped. A tight gasdiameter can now be applied by making use of appropriate surfacecurvatures to deliver the flame shape required or needed. Thestabilizing mechanism of the Coanda surfaces allows that the burner belit successfully at much lower rates of fuel flow. This design alsoallows that the diffusion primary tips be located somewhat deeper in thefurnace for expanded entrainment lengths. Previous designs would notallow a longer entrainment length be utilized without instabilitiesbeing realized. The use of the Coanda surfaces allows that the innerboundary layer remain rich enough to remain combustible. The addition ofthe lean premix ring or distribution header allows that the diffusionprimaries be further flame stabilized by a low NO_(X) homogenous flame.The premix flame allows that the burner turndown be pushed beyondtypical designs without instability being realized. It also allows thatthe burner be highly stable when other burners have been observed tobecome unstable. The combination of the above geometries allows thedesigner of the burner to design a burner of medium range NO_(X), lowNO_(X), or very low NO_(X) within the same basic burner configuration.The stability of the burner is substantially superior to typical naturaldraft or forced draft process burners, allowing the Coanda surfaces toadd additional flue gas into the primary flame zone. Turndown for theburner can now be in excess of 10 to 1 depending on the fuel and theoperational parameters of the burner.

The overall size of the gas burner apparatus in general including thesize of the burner tile 50 can also vary depending on how the apparatusis used. Also, as discussed above, the shape, size, length, height andorientation of the internal and external Coanda surfaces can be adjustedas needed as long as certain other parameters (e.g., a sufficientcurvature) are maintained to achieve a sufficient Coanda effect.

In some applications, the burner tile 50 can be retrofit to existingburner plenums. For example, the burner tile 50 can be retrofitted togas burner apparatus of staged gas design. The burner tile 50 can beadded with new tips and risers to make use of the Coanda approach fordecreased emissions and flame stability. Nitrous oxides can be decreasedin a hot furnace while carbon monoxide can be decreased in a cold box orduring start up.

As shown by FIGS. 22 and 23, the present invention also includes aCoanda gas tip. The tip can be used, for example, as a primary fuel gasdischarge nozzle 168 in connection with the central gas gun 170 (as thebull nose tip) or central venturi mixer 176. It can also be used as aprimary or secondary fuel gas discharge nozzle, or the pilot gas tip230. FIG. 22 illustrates use of the Coanda gas tip as a pilot gas tip.

The inventive Coanda gas tip, generally designated in FIGS. 22 and 23 bythe reference numeral 240, includes a gas barrel 242 for connection to asource of fuel gas (a gas riser, for example), a gas deflector 244attached to the gas barrel, and a fuel gas outlet 246 disposed betweenthe gas barrel and the gas deflector. The gas deflector 244 is attachedto the barrel 242 by an internal threaded connection assembly 248 (othermechanical or welded connections can be used as well). The gas deflector244 has an exterior surface that includes a Coanda surface 250positioned with respect to the fuel gas outlet 246 such that fuel gasdischarged from the fuel gas outlet follows the path of the Coandasurface. The gas deflector 244 of the Coanda gas tip 240 preferably hasa tulip shape which imparts an annular Coanda surface 250 to thedeflector.

In order to achieve a significant Coanda effect, the surface of theCoanda surface 250 should be substantially smooth and have asubstantially true radius or uniform arc. Also, it is important for theCoanda surface 250 to have enough curvature to sufficiently attract thegas stream at issue. If the Coanda surface does not have enoughcurvature or surface area, the surface may not have a sufficient area toinitiate the Coanda effect due to the momentum of the gas (i.e., the gasstream may not be drawn to the surface). In order to assure a sufficientCoanda effect, the ratio of the diameter of the ports of the fuel gasoutlet 246 (if ports are used), or the width of the slots of the fuelgas outlet 246 (if slots are used) (or the average port diameter or slotwidth if multiple ports or slots are used) (the “tip discharge openingdiameter”) to the radius of the Coanda surface 250 (the “tip Coandaradius”) needs to be at least 7:1. Preferably the tip discharge openingdiameter to tip Coanda radius ratio is at least 10:1, most preferably atleast 12:1. Assuming that the Coanda surface 250 has enough curvature orsurface area, the gas stream or jet is aligned to be tangent with thecurvature of the Coanda surface to initiate a proper Coanda effect, evenwhen dealing with small gas ports.

In one embodiment, the fuel gas outlet 246 comprises an annular slot 252which discharges the fuel gas at an appropriate angle (for example, 0 to45°) from the barrel 242, depending on the particular application. Thefuel gas outlet 246 can also comprise a plurality of small circularports (not shown), either in lieu of the slot 252 or in additionthereto. As shown by FIG. 22, in pilot and other applications in whichflame stability is an issue or enhanced mixing is required, a shield 254can be attached to the barrel 242 to surround the deflector 244 andoutlet 246. The shield 254 includes one or more air inlets 260 therein.

The annular Coanda surface 250 of the Coanda gas tip 240 is positionedwith respect to the fuel gas outlet 246 such that fuel gas dischargedfrom the fuel gas outlet follows the path of the Coanda surface. TheCoanda surface spreads the fuel gas into a thin film allowing more airor flue gas or both to be entrained into the fuel gas stream and createa small rapidly mixed three fluid mixture with a fuel rich innerboundary layer for stability. This approach allows the bulk flame toapproach non-combustibility while maintaining a stable flame. The amountof flue gas that can be entrained into the fuel gas stream can beappreciably increased without compromising stability. The overall sizeof the Coanda gas tip 240 including the length and diameter of thebarrel 242 and the size of the deflector 244 can vary depending on thesize of the overall burner and the way the tip is used. For example,when the tip is used as the bull nose tip 168 of the central gas gun170, it is relatively large as compared to its size when it is used asthe pilot tip 230. A smaller size of the tip is typically used whendealing with heat releases of from about 0.05 to about 1.5 MMBtuh. Alarger scale can be used to deliver significantly more fuel gas, forexample when the tip is used as the main injector in the center of thetile (the tip of the central gas gun 170). In this case, the tip candeliver, for example, 3 to 10 million MMBtuh or more if required by theparticular application. The cone and other superfluous componentstypically used in a gas gun are not necessary.

Referring now to FIG. 1, operation of the inventive gas burner apparatus10 will be described. The apparatus 10 is initially lit by an internalpilot or manually ignited by an external torch. Once the pre-mix primaryunit 190 is ignited and up and running, the various header valves 220are opened to supply fuel gas to the remaining burner components. Air isintroduced into the burner housing 22 through the air inlet 32 thereof.The air register or damper 42 regulates the rate of flow of the air intothe housing 22. The air is conducted through the housing 22 anddischarged through the air outlet 30 thereof into the central opening 52of the burner tile 50.

A mixture of primary fuel gas and air is introduced into the centralopening 52 of the burner tile 50 by the pre-mix unit 190. The fuelgas-air mixture is discharged through the pre-mix gas ports 194 aroundthe interior surface 66 of the wall 58 of the burner tile. Primary fuelgas is also injected into the central opening 52 of the burner tile 50by the central gas gun 170. The flow of fuel gas and combustion air isrepresented by the arrows in the drawings. Simultaneously, primary fuelgas is conducted through the outer gas risers 164 and discharged throughthe primary fuel gas discharge nozzles 166 into and through the gascirculation ports 70. Injection of fuel gas from the primary fuel gasdischarge nozzles 166 into the gas circulation ports 70 entrains fluegas from the furnace into the central opening 52 of the burner tile 50.The primary fuel gas and flue gas transported through the ports 70encounter the internal Coanda surfaces 80 and follow the path thereof tothe top end 56 of the burner tile. As stated above, the internal Coandasurfaces 80 cause the fuel gas and flue gas to rapidly mix together andkeep the mixture close to the interior surface 66 of the wall 58 of theburner tile 50 which allows a great deal of flue gas to be entrainedinto the central opening for controlling the temperature of the flameand thereby controlling the emission of nitrous oxides and carbonmonoxide without overly diluting the fuel gas in the central opening 52(for example, to the point on non-combustibility). The mixture ofprimary fuel gas, air and flue gas is ignited by the pre-mix unit 190(or other pilot means) in the central opening 52, discharged through thedischarge outlet 60 and burned in a primary reaction zone 270. Theprimary reaction zone 270 is inside the central opening 52 of the burnertile 50 and outside of burner tile adjacent to the discharge outlet 60thereof.

Secondary stage fuel gas is simultaneously conducted through the outergas risers 164 and discharged through the secondary fuel gas dischargenozzles 168 (which can also be the primary fuel gas discharge nozzles)on or adjacent to the continuous external Coanda surface 130. Thesecondary stage fuel gas follows the path of the external Coanda surface130 to the top end 56 of the burner tile where it is ignited by theflame in the primary combustion zone 170 and is burned in a secondarycombustion zone 280 around and on top of the primary combustion zone.The flow of fuel gas and flue gas with respect to the internal andexternal Coanda surfaces 80 and 130 is best shown by FIGS. 4 and 4A.FIG. 4A illustrates the flow of gas when the circulation choke means 87are utilized to abate the outflow of fluids through the gas circulationports 70.

As shown by FIG. 8, the central venturi mixer 176 can be substituted forthe central gas gun 170 to serve as a quench mechanism for lower nitrousoxide emissions and also to create a shorter flame. As shown by FIG. 9,a plurality of inner gas risers 167 and corresponding fuel gas dischargenozzles 168 can be used instead of or in conjunction with the pre-mixunit 190. The circulation choke means is typically needed when inner gasrisers 167 and nozzles 168 are placed adjacent to the gas circulationports 70 and diffusion fuel gas is not injected through the ports. Asshown by FIGS. 11-16, various configurations of the wall 58 and exteriorsurface 68 (for example, a plurality of external Coanda surfaces 130separated by inclined external planar surfaces 132 or a continuousexternal planar surface 132) can be utilized to achieve a smallerdiameter flame and help control the flame. As shown by FIG. 16-19, thelip 140 can be included in the burner tile 50 to offer additional mixingas well as bluff body stabilization. Finally, different shapes of thegas burner apparatus 10 can be utilized to fit the particularapplication.

Fuel gas is burned in the furnace space 14 at a flow rate which resultsin the desired heat release. The rate of air is introduced into thehousing 22 by way of the air inlet 32 and air register or damper 42 suchthat the desired stoichiometric mixture of fuel gas and air results inthe furnace space 14. That is, a flow rate of air is introduced into thefurnace space 14 relative to the total flow rate of fuel gas introducedthereinto which results in a fuel-air ratio greater than thestoichiometric mixture. Preferably, the rate of air is in the range ofabout 10% to about 25% greater than the stoichiometric rate. The fluegases formed by combustion of the fuel gas in the furnace space 14 havea very low content of nitrous oxides. The portion of the fuel gas whichis used as primary fuel gas is generally in the range of about 5% toabout 25% by volume of the total fuel gas discharged by the burnerapparatus 10 into the furnace space 14. That is, the flow rate ofprimary fuel gas discharged into the furnace space is from about 5% toabout 25% of the total fuel gas flow rate delivered to the burnerapparatus 10 and the flow rate of secondary stage fuel gas discharged isfrom about 95% to about 75% of the total fuel gas flow rate. The primaryfuel gas is mixed with flue gases in an amount in the range of fromabout 1 volume to about 30 volumes of flue gas per volume of the primaryfuel gas depending on available pressure, entrainment length, and thesize of the gas circulation ports 70. Staged gas can be biased to almostany percentage between the primary ports and the staged riser stagedports to optimize heat flux. The heat release of the burner in questionwill dictate for the most part the splits utilized between differentrisers.

In a preferred embodiment, both the internal Coanda surfaces 80 andexternal Coanda surface(s) 130 are utilized. The primary fuel gasinjection means include the outer gas risers 164 and the pre-mix unit190. That is, primary fuel gas is injected into the burner tile 50through the gas circulation ports 70 and above the pre-mix unit 190. Inanother preferred embodiment, both the internal Coanda surfaces 80 andexternal Coanda surface(s) 130 are utilized. However, the primary fuelgas injection means could consist of only the pre-mix unit 190. That is,the only source of primary fuel gas is the pre-mix unit 190. Thedischarge of fuel gas and air from the pre-mix unit 190 and the flow ofair through the central opening 52 would still entrain flue gas into thegas circulation ports 80 into the central opening even though primaryfuel gas is not injected through the gas circulation ports. Flue gasentrained by air flow through the burner will still flow through therecirculation ports in the tile after which a large portion of the fluegas will adhere to the Coanda surface located on the inside.

The invention also provides a method of burning a mixture of air andfuel gas in the presence of flue gas in a furnace to generate heat inthe furnace. The method includes the following steps:

First, the inventive gas burner apparatus is installed through a wall ofthe furnace space (preferably, the bottom wall or floor of the furnacespace). As described above, a plurality of gas circulation ports 70extend through the wall 58 of the burner tile 50. The interior surface66 of the wall 58 includes a plurality of internal Coanda surfaces 80,each internal Coanda surface being positioned adjacent to the gascirculation port 70. Depending on the application, the gas burnerapparatus 10 can also include one or more of the other componentsdescribed above.

Air is injected into the central opening 52 of the burner tile 50.Primary fuel gas is injected through the gas circulation ports 70 on oradjacent to the internal Coanda surfaces 80 to entrain flue gas fromoutside of the wall 58 (for example, from the furnace space) into thecentral opening 52 of the burner tile 50 and form a homogenous mixtureof air, fuel gas and flue gas in the central opening. The mixture ofair, fuel gas and flue gas is discharged from the discharge outlet 60 ofthe top end 56 of the burner tile 50 into the furnace space 14, and themixture of air and fuel gas is burned in the furnace space while heavilydiluted with the furnace flue gas.

In another embodiment, the method of burning a mixture of air and fuelgas in the presence of flue gas in a furnace to generate heat in thefurnace comprises the following steps:

The inventive gas burner 10 is installed through a wall of the furnacespace 14 (preferably a bottom wall or floor of the furnace space 14).The exterior surface 68 of the wall 58 of the burner tile 50 includes anexternal Coanda surface 130 which extends outwardly from the exteriorsurface.

Air and fuel gas are injected into the central opening 52 of the burnertile 50 whereby a mixture of air and fuel gas is formed in the centralopening. The mixture of air and fuel gas is then discharged from thedischarge outlet 60 of the burner tile 50 into the furnace space 14, andthe mixture is burned in a primary reaction zone 270 in the furnacespace. Staged fuel gas is also injected on or adjacent to the externalCoanda surface 130 in a manner that entrains flue gas from the furnacespace 14 to create a staged fuel gas/flue gas mixture and causes thestaged fuel gas/flue gas mixture to burn in a secondary reaction zone280 in the furnace space.

If desired, the steps of the methods described above can be combinedinto a single method.

In order to further illustrate the invention, the following example isprovided.

EXAMPLE

The inventive gas burner apparatus 10 was tested for performance. Theinternal Coanda surfaces 80 and a continuous Coanda surface 130 wereincluded on the wall 58 of the burner tile 50. The primary fuel gasinjection means in the particular burner configuration tested includedthe outer gas risers 164 and fuel gas discharge nozzles 166. The fuelgas discharge nozzles included both ports for injecting primary fuel gasthrough the gas circulation ports 80 and ports for injecting secondaryfuel gas on or adjacent to the external Coanda surface 130. The pre-mixunit 190 was also utilized to reduce nitrous oxide emissions. Thepre-mix membrane 192 included 36 pre-mix gas ports 194 that had a 0.261inch diameter. These ports were spaced around the top surface of thepre-mix membrane 192. Each 0.261 inch port had a 0.125 inch port locatedbetween it that was also counter-bored with a 0.125 inch diameter portsuperimposed over it. The purpose of the smaller ports was to serve asan ignition port which was utilized to tie together the larger ports.Neither inner gas risers 167, the central gas gun 170 nor the centralventuri mixer 176 were utilized. Generally, the gas burner apparatus 10tested was configured like the gas burner apparatus 10 shown in FIGS.1-7 except the central gas gun 170 was not included.

The pre-mix unit was manually ignited followed by the ignition of therest of the burner. The damper 42 was left all the way open during alltest points. The pre-mix primary unit lit nicely creating a uniform setof blue flamelets around the internal perimeter of the burner tile. Themain portion of the burner was then lit with a pressure of approximately0.1 psig. The burner was then increased in heat release to roughly 0.84MMBtuh to start warming the furnace. The flame was stiff and appearedvery stable. Carbon monoxide and nitrous oxide levels were very good atall test points maintaining recordable emissions of less than 26 ppmv(avg) from light off to saturation. The burner tile 50 was observed tobe glowing red through all the testing.

The following test data was generated.

Test Data Heat Release 0.85 MMBtuh Tip Pressure 0.4 psig Fuel Gas 100%TNG* Spud Size #52 MTD Pre-Mix Gas — NO_(X) Emissions 5.31 ppmv COEmissions 34.80 ppmv Percent O2 18.63% Flame Quality very good Mixertype Std. Brnr. Pilot Pre-mix Tip (Large Ports) 0.261″ Pre-mix Tip(Small Ports) 0.125″ Furnace Floor Temp 336° F. Furnace Temp 384° F.Heat Release 2.07 MMBtuh Tip Pressure 2.6 psig Fuel Gas 100% TNG* SpudSize #52 MTD Pre-Mix Gas — NO_(X) Emissions 11.2 ppmv CO Emissions 9.04ppmv Percent O2 16.15 Flame Quality very good Mixer type Std. Brnr.Pilot Pre-mix Tip (Large Ports) 0.261″ Pre-mix Tip (Small Ports) 0.125″Furnace Floor Temp 683° F. Furnace Temp 717° F. Heat Release 3.0 MMBtuhTip Pressure 5.4 psig Fuel Gas 100% TNG* Spud Size #52 MTD Pre-Mix Gas —NO_(X) Emissions 12.42 ppmv CO Emissions 12.33 ppmv Percent O2 14.38%Flame Quality very good Mixer type Std. Brnr. Pilot Pre-mix Tip (LargePorts) 0.261″ Pre-mix Tip (Small Ports) 0.125″ Furnace Floor Temp 859°F. Furnace Temp 893° F. Heat Release 4.00 MMBtuh Tip Pressure 9.4 psigFuel Gas 100% TNG* Spud Size #52 MTD Pre-Mix Gas — NO_(X) Emissions10.19 ppmv CO Emissions 26.62 ppmv Percent O2 12.56% Flame Quality verygood Mixer type Std. Brnr. Pilot Pre-mix Tip (Large Ports) 0.261″Pre-mix Tip (Small Ports) 0.125″ Furnace Floor Temp 1015° F. FurnaceTemp 1036° F. Heat Release 4.97 MMBtuh Tip Pressure 15.3 psig Fuel Gas85% TNG* and 15% H2 Spud Size #52 MTD Pre-Mix Gas — NO_(X) Emissions9.95 ppmv CO Emissions 10.99 ppmv Percent O2 10.22% Flame Quality verygood Mixer type Std. Brnr. Pilot Pre-mix Tip (Large Ports) 0.261″Pre-mix Tip (Small Ports) 0.125″ Furnace Floor Temp 1138° F. FurnaceTemp 1161° F. Heat Release 6.01 MMBtuh Tip Pressure 20.9 psig Fuel Gas85% TNG*/15% H2 Spud Size #52 MTD Pre-Mix Gas — NO_(X) Emissions 10.74ppmv CO Emissions 9.30 ppmv Percent O2 8.12% Flame Quality Very goodMixer type Std. Brnr. Pilot Pre-mix Tip (Large Ports) 0.261″ Pre-mix Tip(Small Ports) 0.125″ Furnace Floor Temp 1216° F. Furnace Temp 1256° F.Heat Release 6.50 MMBtuh Tip Pressure 23.6 psig Fuel Gas 85% TNG*/15% H2Spud Size #52 MTD Pre-Mix Gas — NO_(X) Emissions 12.99 ppmv CO Emissions1.10 ppmv Percent O2 7.01% Flame Quality Very good Mixer type Std. Brnr.Pilot Pre-mix Tip (Large Ports) 0.261″ Pre-mix Tip (Small Ports) 0.125″Furnace Floor Temp 1242° F. Furnace Temp 1322° F. Heat Release 7.04MMBtuh Tip Pressure 26.7 psig Fuel Gas 85% TNG*/15% H2 Spud Size #52 MTDPre-Mix Gas — NO_(X) Emissions 13.66 ppmv CO Emissions 0.00 ppmv PercentO2 5.63% Flame Quality Very good Mixer type Std. Brnr. Pilot Pre-mix Tip(Large Ports) 0.261″ Pre-mix Tip (Small Ports) 0.125″ Furnace Floor Temp1271° F. Furnace Temp 1367° F. Heat Release 7.28 MMBtuh Tip Pressure28.1 psig Fuel Gas 85% TNG*/15% H2 Spud Size #52 MTD Pre-Mix Gas —NO_(X) Emissions 13.37 ppmv CO Emissions 0.00 ppmv Percent O2 4.68%Flame Quality Very good Mixer type Std. Brnr. Pilot Pre-mix Tip (LargePorts) 0.261″ Pre-mix Tip (Small Ports) 0.125″ Furnace Floor Temp 1283°F. Furnace Temp 1376° F. Heat Release 7.98 MMBtuh Tip Pressure 31.9 psigFuel Gas 85% TNG*/15% H2 Spud Size #52 MTD Pre-Mix Gas — NO_(X)Emissions 11.32 ppmv CO Emissions 0.00 ppmv Percent O2 2.56% FlameQuality Very good Mixer type Std. Brnr. Pilot Pre-mix Tip (Large Ports)0.261″ Pre-mix Tip (Small Ports) 0.125″ Furnace Floor Temp 1294° F.Furnace Temp 1469° F. Heat Release 8.10 MMBtuh Tip Pressure 32.4 psigFuel Gas 85% TNG*/15% H2 Spud Size #52 MTD Pre-Mix Gas — NO_(X)Emissions 10.82 ppmv CO Emissions 0.00 ppmv Percent O2 1.93% FlameQuality Very good Mixer type Std. Brnr. Pilot Pre-mix Tip (Large Ports)0.261″ Pre-mix Tip (Small Ports) 0.125″ Furnace Floor Temp 1286° F.Furnace Temp 1475° F. Heat Release 8.33 MMBtuh Tip Pressure 34.0 psigFuel Gas 85% TNG*/15% H2 Spud Size #52 MTD Pre-Mix Gas — NO_(X)Emissions 10.24 ppmv CO Emissions 0.00 ppmv Percent O2 2.08% FlameQuality Very good Mixer type Std. Brnr. Pilot Pre-mix Tip (Large Ports)0.261″ Pre-mix Tip (Small Ports) 0.125″ Furnace Floor Temp 1282° F.Furnace Temp 1499° F. Heat Release 8.58 MMBtuh Tip Pressure 35.1 psigFuel Gas 85% TNG*/15% H2 Spud Size #52 MTD Pre-Mix Gas — NO_(X)Emissions 10.34 ppmv CO Emissions 0.00 ppmv Percent O2 0.67% FlameQuality Very good Mixer type Std. Brnr. Pilot Pre-mix Tip (Large Ports)0.261″ Pre-mix Tip (Small Ports) 0.125″ Furnace Floor Temp 1282° F.Furnace Temp 1532° F. Heat Release 8.62 MMBtuh Tip Pressure 35.3 psigFuel Gas 85% TNG*/15% H2 Spud Size #52 MTD Pre-Mix Gas — NO_(X)Emissions 9.71 ppmv CO Emissions 2.44 ppmv Percent O2 0.37% FlameQuality Very good Mixer type Std. Brnr. Pilot Pre-mix Tip (Large Ports)0.261″ Pre-mix Tip (Small Ports) 0.125″ Furnace Floor Temp 1284° F.Furnace Temp 1537° F. Heat Release 8.65 MMBtuh Tip Pressure 35.3 psigFuel Gas 85% TNG*/15% H2 Spud Size #52 MTD Pre-Mix Gas — NO_(X)Emissions 9.22 ppmv CO Emissions 131.8 ppmv Percent O2 0.15% FlameQuality Good Mixer type Std. Brnr. Pilot Pre-mix Tip (Large Ports)0.261″ Pre-mix Tip (Small Ports) 0.125″ Furnace Floor Temp 1283° F.Furnace Temp 1501° F. *Tulsa Natural Gas

Thus, the inventive gas burner apparatus performed very well. Thepre-mix unit 190 worked well. The carbon monoxide observed during lightoff, warm up and stable running was for the most part non-existent.Nitrous oxide emissions were also observed to be very low.

1. A gas burner apparatus for discharging a mixture of fuel gas and airinto a furnace wherein the mixture is burned in the presence of flue gaswhile producing a low content of nitrous oxides comprising: a housingfor attachment to said furnace, said housing including: an upper endattached to said furnace, said upper end having an air outlet disposedtherein; a lower end opposing said upper end; and a sidewall connectingsaid upper end and said lower end together, wherein at least one of saidsidewall and said lower end has an air inlet disposed therein; a burnertile having a central opening therein for receiving air from saidcombustion air outlet of said housing, said tile including: a bottom endattached to said upper end of said housing over said air outlet; a topend opposing said bottom end, said top end including a discharge outlet;and a wall connecting said bottom end to said top end and surroundingsaid central opening, said wall extending into said furnace and havingan interior surface, an exterior surface and at least one gascirculation port extending though said wall, said interior surface ofsaid wall including an internal Coanda surface which bulges into saidcentral opening; primary fuel gas injection means connected to a sourceof fuel gas and operably associated with said burner apparatus forinjecting primary fuel gas into said central opening of said burnertile, said primary fuel gas injection means including a pre-mix unitcomprising: a pre-mix membrane extending around the interior surface ofsaid wall of said burner tile below said gas circulation port thereinand having a plurality of pre-mix gas ports in the top thereof; and aventuri mixer including an inner gas riser connected to said source offuel gas and having an inner primary fuel gas discharge nozzle connectedthereto, and a venturi housing operably associated with said inner gasriser and primary fuel gas discharge nozzle and connected to saidpre-mix membrane for feeding a mixture of primary fuel gas and air intosaid pre-mix membrane; and secondary fuel gas injection means connectedto a source of fuel gas and operably associated with said burnerapparatus for injecting secondary stage fuel gas from outside of saidburner tile to a point adjacent to said discharge outlet of said burnertile.
 2. The gas burner apparatus of claim 1 wherein said burner tilefurther includes circulation choke means positioned in said gascirculation port for inhibiting the flow of fluids from within saidcentral opening of said tile through said gas circulation port.
 3. Thegas burner apparatus of claim 2 wherein said circulation choke meansincludes a shield attached to said wall of said burner tile andextending upwardly into said gas circulation port.
 4. A burner tile foruse in association with a burner plenum to form a gas burner apparatusfor discharging a mixture of fuel gas and air into a furnace wherein themixture is burned in the presence of flue gas while producing a lowcontent of nitrous oxides, wherein the burner plenum includes a housingfor attachment to the furnace that includes an upper end having an airoutlet disposed therein, said burner tile having a central openingtherein for receiving air from the outlet of the plenum housing andcomprising: a bottom end attachable to the upper end of the plenumhousing over the air outlet disposed therein; a top end opposing saidbottom end, said top end including a discharge outlet; and a wallconnecting said bottom end to said top end and surrounding said centralopening, said wall extendable into said furnace and having an interiorsurface, an exterior surface and at least one gas circulation portextending through said wall, said interior surface of said wallincluding an internal Coanda surface which bulges into said centralopening.
 5. The burner tile of claim 4 wherein said internal Coandasurface is positioned on said interior surface of said wall adjacent tosaid gas circulation port.
 6. The burner tile of claim 5 wherein saidinterior surface of said wall of said tile includes a recessed section,and said gas circulation port and said internal Coanda surface arepositioned in said recessed section.
 7. The burner tile of claim 6wherein said recessed section includes opposing sidewalls extendingoutwardly from said interior surface into said central opening of saidtile, said opposing sidewalls extending further into said centralopening than said internal Coanda surface extends into said centralopening.
 8. A burner tile for use in association with a burner plenum toform a gas burner apparatus for discharging a mixture of fuel gas andair into a furnace wherein the mixture is burned in the presence of fluegas while producing a low content of nitrous oxides, wherein the burnerplenum includes a housing for attachment to the furnace that includes anupper end having an air outlet disposed therein, said burner tile havinga central opening therein for receiving air from the air outlet of theplenum housing and comprising: a bottom end attachable to the upper endof the plenum housing over the air outlet disposed therein; a top endopposing said bottom end, said top end including a discharge outlet; anda wall connecting said bottom end to said top end and surrounding saidcentral opening, said wall extendable into said furnace and having aninterior surface and an exterior surface, said exterior surface of saidwall including an external Coanda surface which bulges outwardly fromsaid exterior surface.
 9. The burner tile of claim 8 wherein saidexterior surface of said wall of said burner tile includes a pluralityof external Coanda surfaces, each of said external Coanda surfacesbulging outwardly from said exterior surface.
 10. The burner tile ofclaim 9 wherein said external Coanda surface extends completely aroundsaid exterior surface of said wall of said burner tile.
 11. A burnertile for use in association with a burner plenum to form a gas burnerapparatus for discharging a mixture of fuel gas and air into a furnacewherein the mixture is burned in the presence of flue gas whileproducing a low content of nitrous oxides, wherein the burner plenumincludes a housing for attachment to the furnace that includes an upperend having an air outlet disposed therein, said burner tile having acentral opening therein for receiving air from the air outlet of theplenum housing and comprising: a bottom end attachable to the upper endof the plenum housing over the air outlet disposed therein; a top endopposing said bottom end, said top end including a discharge outlet; anda wall connecting said bottom end to said top end and surrounding saidcentral opening, said wall extendable into said furnace and having aninterior surface, an exterior surface and at least one gas circulationport extending through said wall, said interior surface of said wallincluding an internal Coanda surface which bulges into said centralopening, said exterior surface of said wall including an external Coandasurface which bulges outwardly from said exterior surface.
 12. Theburner tile of claim 11 wherein said wall of said burner tile includes aplurality of gas circulation ports extending through said wall.
 13. Theburner tile of claim 12 wherein said interior surface of said wallincludes a plurality of internal Coanda surfaces, each of said internalCoanda surfaces bulging into said central opening of said burner tile.14. The burner tile of claim 13 wherein said exterior surface of saidwall of said burner tile includes a plurality of external Coandasurfaces, each of said external Coanda surfaces bulging outwardly fromsaid exterior surface.
 15. The burner tile of claim 13 wherein saidexternal Coanda surface extends completely around said exterior surfaceof said wall of said burner tile.
 16. The burner tile of claim 15wherein said tile has a substantially circular cross-sectional shape.17. A gas tip for use in association with a gas burner apparatus,comprising: a gas barrel for connection to a source of fuel gas; a gasdeflector attached to said gas barrel; and a fuel gas outlet disposedbetween said gas barrel and said gas deflector, said gas deflectorhaving an exterior surface that includes a Coanda surface positionedwith respect to said fuel gas outlet such that fuel gas discharged fromsaid fuel gas outlet follows the path of said Coanda surface.
 18. Amethod of burning a mixture of air, fuel gas and flue gas in a furnacespace of a furnace to generate heat in the furnace space wherein a gasburner apparatus having a mixing zone for mixing the air, fuel gas andflue gas prior to combustion thereof is utilized, comprising: providinga Coanda surface in said mixing zone; injecting fuel gas on or adjacentto said Coanda surface in a manner that entrains flue gas from outsidesaid mixing zone into said mixing zone and causes said flue gas to mixwith said air and fuel gas in said mixing zone; discharging said mixtureof air, fuel gas and flue gas from said mixing zone into said furnacespace; and burning said mixture of combustion air, fuel gas and flue gasdischarged from said mixing zone in said furnace space.
 19. The methodof claim 18 wherein said mixing zone is surrounded by a wall and saidmixture of air, fuel gas and flue gas is discharged from said mixingzone into a primary reaction zone in said furnace space, and whereinsaid method further comprises the steps of: providing an external Coandasurface on the exterior surface of said wall; injecting a stream ofsecondary stage fuel gas on or adjacent to said external Coanda surfacein a manner that entrains flue gas into the stream to create a secondaryfuel gas/flue gas mixture and causes said secondary fuel gas/flue gasmixture to burn in a secondary reaction zone in said furnace space. 20.A method of burning a mixture of air, fuel gas and flue gas in a furnacespace of a furnace to generate heat in the furnace space wherein a gasburner apparatus including a wall surrounding a mixing zone for mixingthe air, fuel gas and flue gas prior to combustion thereof is utilized,comprising: providing a Coanda surface on the exterior surface of thewall of the burner apparatus; injecting primary fuel gas into the mixingzone in a manner that causes the fuel gas to mix with air in the mixingzone; discharging the mixture of air and fuel gas from the mixing zone;burning the mixture of air and fuel gas discharged from the mixing zonein a primary reaction zone in the furnace space; injecting a stream ofsecondary stage fuel gas on or adjacent to the external Coanda surfacein a manner that entrains flue gas into the stream to create a secondaryfuel gas/flue gas mixture and causes such secondary fuel gas/flue gasmixture to burn in a secondary reaction zone in the furnace space. 21.The method of claim 20 wherein the interior surface of the wall of theburner apparatus includes an internal Coanda surface, and the fuel gasinjected into the mixing zone is injected on or adjacent to the internalCoanda surface in a manner that entrains flue gas from outside themixing zone into the mixing zone and causes the flue gas to mix with theair and fuel gas in the mixing zone.
 22. A gas burner apparatus fordischarging a mixture of fuel gas and air into a furnace wherein themixture is burned in the presence of flue gas while producing a lowcontent of nitrous oxides, comprising: a plenum including a housing forattachment to said furnace, said housing including: an upper endattached to said furnace, said upper end having an air outlet disposedtherein; a lower end opposing said upper end; and a sidewall connectingsaid upper end and said lower end together, wherein at least one of saidsidewall and said lower end has an air inlet disposed therein; a burnertile having a central opening therein for receiving air from said airoutlet of said housing, said tile including: a bottom end attached tosaid upper end of said housing over said air outlet; a top end opposingsaid bottom end, said top end including a discharge outlet; and a wallconnecting said bottom end to said top end and surrounding said centralopening, said wall extending into said furnace and having an interiorsurface and an exterior surface; and primary fuel gas injection meansconnected to a source of fuel gas and operably associated with saidburner apparatus for injecting primary fuel gas into said centralopening of said burner tile, said primary fuel gas injection meansincluding a pre-mix unit, said pre-mix unit including: a pre-mixmembrane extending around the interior surface of said wall of saidburner tile for injecting a pre-mixed stream of primary fuel gas and airaround the interior surface of said wall and around said central openingof said burner tile, said pre-mix membrane including at least one inletand at least one outlet therein; and a venturi mixer including an innergas riser connected to a source of fuel gas and having an inner primaryfuel gas discharge nozzle connected thereto, and a venturi housingoperably associated with said inner gas riser and primary fuel gasdischarge nozzle and connected to said inlet of said pre-mix membranefor feeding a mixture of primary fuel gas and air into said pre-mixmembrane.
 23. The gas burner apparatus of claim 22 wherein said pre-mixmembrane extends around the interior surface of said wall of said burnertile in a manner that allows it to symmetrically inject a pre-mixedstream of primary fuel gas and air around the interior surface of saidwall and around said central opening of said burner tile.
 24. The gasburner apparatus of claim 22 wherein said pre-mix membrane extendscontinuously around the interior surface of said wall of said burnertile.
 25. The gas burner apparatus of claim 22 wherein said pre-mixmembrane is inset somewhat in the interior surface of said wall of saidburner tile.
 26. The gas burner apparatus of claim 22 wherein saidpre-mix membrane includes a plurality of pre-mix gas ports in the topthereof.
 27. The gas burner apparatus of claim 22 wherein said pre-mixmembrane includes two inlets and said pre-mix unit includes two venturimixers, each venturi mixer including an inner gas riser connected tosaid source of fuel gas and having an inner primary fuel gas dischargenozzle connected thereto, and a venturi housing operably associated withsaid inner gas riser and primary fuel gas discharge nozzle and connectedto one of said inlets of said pre-mix membrane for feeding a mixture ofprimary fuel gas and air into said pre-mix membrane.
 28. The gas burnerapparatus of claim 22, wherein said primary fuel gas injection meansfurther includes a primary fuel gas injection assembly separate fromsaid pre-mix unit for injecting primary fuel gas into said centralopening of said burner tile.
 29. The gas burner apparatus of claim 28wherein said primary fuel gas injection assembly includes an inner gasriser connected to a source of fuel gas and positioned inside of saidhousing, said inner gas riser having an inner primary fuel gas dischargenozzle connected thereto for injecting primary fuel gas into saidcentral opening of said burner tile.
 30. The gas burner apparatus ofclaim 28 wherein said primary fuel gas injection assembly includes aninner gas riser connected to a source of fuel gas and positioned insideof said housing, said inner gas riser having an inner primary fuel gasdischarge nozzle connected thereto and a venturi housing operablyassociated therewith for injecting a mixture of primary fuel gas and airinto said central opening of said burner tile.
 31. The gas burnerapparatus of claim 28 wherein said pre-mix unit and said primary fuelgas injection assembly are connected to separate sources of fuel gas.32. The gas burner apparatus of claim 22, further comprising secondaryfuel gas injection means connected to a source of fuel gas and operablyassociated with said burner apparatus for injecting secondary stage fuelgas from outside of said burner tile to a point adjacent to saiddischarge outlet of said burner tile.
 33. The gas burner apparatus ofclaim 32 wherein said secondary fuel gas injection means includes anouter gas riser connected to said source of fuel gas and having asecondary fuel gas discharge nozzle connected thereto for injectingsecondary stage fuel gas on or adjacent to said exterior surface of saidwall of said burner tile.
 34. The gas burner apparatus of claim 22wherein said burner tile and said pre-mix membrane of said pre-mix unithave substantially round cross-sectional shapes.
 35. The gas burnerapparatus of claim 22 wherein said burner tile further comprises a liptransversely extending from said interior surface of said wall into saidcentral opening, said lip being attached to said wall adjacent to saidtop end of said burner tile and extending around said interior surfaceof said wall of said burner tile.
 36. The gas burner apparatus of claim35 wherein said lip includes a lower end, a top end and a bodyconnecting said lower end and said top end together, said body includinga plurality of protrusions extending into said central opening.
 37. Aburner tile for use in association with a burner plenum to form a gasburner apparatus for discharging a mixture of fuel gas and air into afurnace wherein the mixture is burned in the presence of flue gas whileproducing a low content of nitrous oxides, wherein the burner plenumincludes a housing for attachment to the furnace that includes an upperend having an air outlet disposed therein, said burner tile having acentral opening therein for receiving air from the outlet of the plenumhousing and comprising: a bottom end attachable to the upper end of saidplenum housing over the air outlet disposed therein; a top end opposingsaid bottom end, said top end including a discharge outlet; and a wallconnecting said bottom end to said top end and surrounding said centralopening, said wall extendable into said furnace and having an interiorsurface and an exterior surface; and a pre-mix membrane extending aroundthe interior surface of said wall for injecting a pre-mixed stream ofprimary fuel gas and air around the interior surface of said wall andaround said central opening, said pre-mix membrane including at leastone inlet for connection to a source of fuel gas and air and at leastone outlet therein,
 38. The burner tile of claim 37 wherein said pre-mixmembrane extends around the interior surface of said wall in a mannerthat allows it to symmetrically inject a pre-mixed stream of primaryfuel gas and air around the interior surface of said wall and aroundsaid central opening.
 39. The burner tile of claim 37 wherein saidpre-mix membrane extends continuously around the interior surface ofsaid wall.
 40. The burner tile of claim 37 wherein said pre-mix membraneis inset somewhat in the interior surface of said wall of said burnertile.
 41. The burner tile of claim 37 wherein said pre-mix membraneincludes a plurality of pre-mix gas ports in the top thereof.
 42. Theburner tile of claim 37 wherein said burner tile and said pre-mixmembrane have substantially round cross-sectional shapes.
 43. The burnertile of claim 37 wherein said burner tile further comprises a liptransversely extending from said interior surface of said wall into saidcentral opening, said lip being attached to said wall adjacent to saidtop end of said burner tile and extending around said interior surfaceof said wall of said burner tile.
 44. The burner tile of claim 43wherein said lip includes a lower end, a top end and a body connectingsaid lower end and said top end together, said body including aplurality of protrusions extending into said central opening.
 45. Amethod of burning a mixture of air, fuel gas and flue gas in a furnacespace of a furnace to generate heat in the furnace space wherein a gasburner apparatus including a wall surrounding a mixing zone for mixingthe air, fuel gas and flue gas prior to combustion thereof is utilized,comprising: providing a pre-mix membrane in said mixing zone, saidpre-mix membrane extending around the interior surface of said wall andthe perimeter of said mixing zone; discharging a mixture of fuel gas andair from said pre-mix membrane around the perimeter of said mixing zone;igniting said mixture of fuel gas and air discharged from said pre-mixmembrane around the perimeter of said mixing zone; injecting primaryfuel gas into said mixing zone from a primary fuel gas injectionassembly separate from said pre-mix unit in a manner that entrains fluegas from outside said mixing zone into said mixing zone and causes saidflue gas to mix with air and fuel gas in said mixing zone; using saidignited mixture of fuel gas and air discharged from said pre-mixmembrane around the perimeter of said mixing zone to ignite the mixtureof primary fuel gas, flue gas and air formed by injecting primary fuelgas into said mixing zone from said primary fuel gas injection assembly;discharging said mixture of air, fuel gas and flue gas from said mixingzone into said furnace space; and burning said mixture of combustionair, fuel gas and flue gas discharged from said mixing zone in saidfurnace space.
 46. The method of claim 45 wherein said mixture of air,fuel gas and flue gas is discharged from said mixing zone into a primaryreaction zone in said furnace space, and wherein said method furthercomprises the steps of: injecting a stream of secondary stage fuel gason or adjacent to the exterior surface of said wall in a manner thatentrains flue gas into the stream to create a secondary fuel gas/fluegas mixture and causes said secondary fuel gas/flue gas mixture to burnin a secondary reaction zone in said furnace space.